If you go to this link, you'll see one person's personal experience with Bloody mary that is apparently true. I once asked her about it and she told me that it actually happened and warned me to never do Bloody Mary.

Are you talking about the story and who its based off? Or the ghost story that's based off it? I personally don't think the ghost story is real, only because I believe in reincarnation and by now she would have been reincarnated. If there is any truth in it, than its probably some other spirit messing with people and scaring them. Than again it depends on what ghost story your talking about...are you talking about saying her name while looking in the mirror and her face appears? Or the more gruesome ones? I could believe the mirror one though again I don't think its her, the others I'd have to question.

Bloody mary (we're not talking about the drink lol) is a folklore legend consisting of a ghost or spirit conjured to reveal the future. She is said to appear in a mirror when her name is called three times. other stories say that she will drag you into the mirror or come out of the mirror and kill you.

"A woman called Countess Elizabeth Bathory Nadasdy ( 1560 - 1614 ) is famous as a real historical figure who was reputed to have not only drunk but bathed in the blood of young virgin girls in order to retain her youth. The Bloody Mary legend could have been based on the actions of this evil and cruel woman. The next contender is Queen Mary Tudor ( 1516 - 1558 ). Of course she was not given this nickname until after her death. Queen Mary Tudor was a Catholic who was responsible for the deaths of English protestants. These protestants were burnt at the stake, the favoured method of execution by the Spanish Inquisition. The threat of the Spanish inquisition terrified the English. The inquisition was a real threat as Spain launched an invasion plan with the Spanish Armada in 1588. It was defeated but the English feared a Catholic monarch and severely blackened the name of Queen Mary Tudor because of this. Countess Elizabeth Bathory Nadasdy was alive during the Tudor period. Combine the stories of the two women and throw in some myths about witches and you probably get close to the basis of the Bloody Mary legend. "

Source:http://www.sixwives.info/bloody-mary-legend.htm

That is one idea about where the Bloody Mary legend came from. It is possible that it is a made up legend, or that it has soome turth to it.

Though I would use caution trying to summon anything by use of incantation, candle, and mirror. You might actually reach a malevolent spirit oif you aren't careful.